1. Field of the Invention
The invention generally relates to tempered glasses. More particularly, the invention relates to thermally tempered glass elements and use thereof.
2. Description of Related Art
Nowadays, borosilicate glasses with low CTE (about 3.3 ppm/K) are being employed as windows in ovens with pyrolysis cleaning function. Because of the low coefficient of linear thermal expansion (CTE), the glass pane has a certain temperature shock resistance. The high breaking strength required for the application as a viewing window may be achieved by thermally tempering the glasses. For this purpose, the glasses are heated above glass transition temperature Tg and quenched. The amount the compressive stress introduced thereby depends on the temperature expansion coefficient and the temperature gradient during the tempering process. The standard borosilicate glass that is usually employed has to be heated to above 750° C. in order to achieve a significant compressive stress (about 60 MPa).
A drawback of the prior art is that after the first pyrolysis cleaning cycle borosilicate glasses with low CTE (about 3.3 ppm/K) lose about 70% of their initial compressive stress by relaxation. Breaking strength also decreases correspondingly. In addition, the relaxation of the glass structure causes so-called compaction effects, that is a change in density which leads to deformations of the glass panes. Additional drawbacks are on the one hand the high temperatures which are necessary for tempering low CTE borosilicate glasses. For example, a temperature of more than 750° C. is required for tempering a borosilicate glass that has a low coefficient of thermal expansion, whereas for soda-lime glasses a temperature of 650° C. is already sufficient to achieve a sufficiently high thermal prestress and hence breaking strength. On the other hand, the low CTE of such a glass is disadvantageous, as it greatly limits the amount of compressive stress that can be introduced using standard tempering furnaces.
DE 4 325 656 C2 describes the production of a tempered glass body that is suitable as a fire safety glass, in a conventional air tempering system. The glass body has a coefficient of thermal expansion between 3 and 6*10−6 K−1, specific thermal stress between 0.3 and 0.5 N/mm2/K, glass transition temperature Tg is between 535° C. and 850° C., and the temperature T13 at which the glass has a viscosity of 1013 dPa·s must be more than 560° C. The softening point according to Littleton, T7.6, must be above 830° C., and the working point T4 at which the glass has a viscosity of 104 dPa·s must be below 1300° C. The glass has a composition, in percent by weight on an oxide basis, of 73 to 78, preferably 57 to 64 for SiO2, 11 to 18 for Al2O3, 5 to 10 for MgO, 5 to 10 for CaO, 9 to 12 of B2O3, and the sum of components MgO, CaO, SrO, BaO, ZnO, ZrO2 is in a range from 6 to 10 wt %.
WO 2015/009483 A1 describes an alkali-free aluminosilicate glass having a composition, in percent by weight on an oxide basis, of 60 to 70 for SiO2, 13 to 22 for Al2O3, 0 to 9 for B2O3, 1 to 6 for MgO, 0 to 5 for CaO, 1 to 5 for BaO, 2 to 12 for ZnO, and 0 to 3 for SrO, with a total content of Al2O3+B2O3+ZnO>23 and with the following relationship applying: B2O3+MgO−CaO−BaO−SrO<6 wt %. The glass is said to have low thermal expansion and great acid and alkali resistance and to be usable as a cooktop. The glass is said to exhibit low thermal expansion of <30*10−7 K−1. With these parameters, however, this glass does not lend itself to be thermally tempered, or only slightly. Furthermore, the high content of Al2O3 typically causes high T13 values which do not allow for the desired tempering in a standard tempering furnace. Moreover, high Al2O3 contents usually reduce acid resistance.
Furthermore, US 2005/145241 A describes a door of a cooking appliance with pyrolysis function, the door comprising a borosilicate glass pane. The glass pane is coated with a colored layer at least in one surface area. The colored layer is adapted so that the strength of the glass pane is not affected by the coating, i.e. in particular not reduced.
From US 2015/107575 A1, an oven door is known which comprises an outer and an inner glass pane having a composition, on an oxide basis, of 55 to 70 wt % of SiO2, 12 to 25 wt % of Al2O3, 0 to 0.5 wt % of B2O3, 0 to 2 wt % of Li2O, 0 to 5 wt % of Na2O+K2O, 0 to 10 wt % of MgO, from 0 to 15 wt % of CaO, of SrO, and of BaO, 0 to 5 wt % of ZnO, and 5 to 25 of RO, with RO=MgO+CaO+SrO+BaO+ZnO, 0 to 3 wt % of TiO2, and 0 to 4 wt % of ZrO2.
These glasses known from prior art do not yet provide any solution for the problem of providing permanent prestress in a glass pane even at temperatures which occur during pyrolysis cleaning, which glass pane should moreover exhibit high chemical resistance. Chemical resistance is particularly important especially in applications in an oven under the temperatures prevailing there during operation and given the higher chemical reactivity resulting therefrom. The prestress, i.e. compressive stress, should furthermore be easy to produce. This means that the glass should be capable of being sufficiently tempered with the temperature gradients that can be produced in a conventional tempering furnace.